Turbochargers are a type of forced induction system. They deliver air, at greater density than would be possible in the normally aspirated configuration, to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. A smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, will reduce the mass and can reduce the aerodynamic frontal area of the vehicle.
Referring to FIG. 1, a turbocharger (10) uses the exhaust flow from the engine exhaust manifold to drive a turbine wheel (12), which is located in a turbine housing (14) to form a turbine stage (16). The energy extracted by turbine wheel (12) is translated into a rotating motion which then drives a compressor wheel (18), which is located in a compressor cover (20), to form a compressor stage (22). The compressor wheel (18) draws air into the turbocharger (10), compresses this air, and delivers it to the intake side of the engine. The turbocharger (10) has an associated axis (11).
Variable Geometry turbochargers typically use a plurality of rotatable vanes (24) to control the flow of exhaust gas, which impinges on the turbine wheel (12) and controls the power of the turbine stage (16). These vanes (24) also therefore control the pressure ratio generated by the compressor stage (22). In engines, which control the production of NOx by the use of High Pressure Exhaust Gas Recirculation (HP EGR) techniques, the function of the vanes (24) in a VTG also provides a means for controlling and generating exhaust back pressure.
An array of pivotable vanes (24) is located between a generally annular upper vane ring (UVR) (26) and a generally annular lower vane ring (LVR) (28). Each vane rotates on a pair of opposing axles (30) (FIGS. 2A and 2B), protruding from said vane (24) with the axles on a common axis. Each axle (30) is located in a respective aperture in the LVR (20) and a respective aperture in the UVR (30). The angular orientation of the UVR (26), relative to the LVR (20), is set such that the complementary apertures in the vane rings (26, 28) are concentric with the axis of the axles (30) of the vane (24), and the vane (24) is free to rotate about the axis (32) of the two axles (30), which is concentric with the now established centerline of the two apertures. Each axle (30) on the UVR side of the vane (24) protrudes through the UVR (26) and is affixed to a vane arm (34), which controls the rotational position of the vane (24) with respect to the vane rings (26, 28). Typically, there is a separate ring which controls all of the vane arms (34) in unison via small sliding blocks (48). This unison ring (50) is controlled by an actuator which is operatively connected to rotate the unison ring (50). The actuator is typically commanded by the engine electronic control unit (ECU). The assembly consisting of the plurality of vanes (24) and the two vane rings (26, 28) is typically known as the vane pack. Typically there is a separate ring which controls all of the vane arms in unison via small sliding blocks (48).
Because the turbine housing (14) is not symmetrically round in a radial plane, and because the heat flux within the turbine housing (14) is also not symmetrical, the turbine housing (14) is subject to asymmetric stresses and asymmetric thermal deformation.
The clearance between the rotatable vanes (24), more specifically between the cheeks (36) of the vanes (24) and the inner surfaces (38, 40) of the upper and lower vane rings (26, 28), is a major contributor to a loss of efficiency in both the control of exhaust gas allowed to impinge on the turbine wheel (12) and in the generation of backpressure upstream of the turbine wheel (12). The clearances between the vane side cheeks (36) and the complementary inner surfaces (38, 40) of the vane rings (26, 28) are kept to a minimum to increase the efficiency of the vane pack.
Unfortunately, the increase in efficiency due the side clearances is inversely proportional to the propensity of the vane pack to wear, stick, or completely jam due to thermal deformation in the turbine housing (14) being transferred to the vane pack. Thus, the vane pack needs to be accurately placed and constrained within the turbine housing (14) in a manner which minimizes the transference of thermally induced distortion. While internal to the vane pack, the aforementioned clearances can be sized to maximize efficiency while minimizing the potential for sticking, jamming, and wear.
In some VTGs, as depicted in FIG. 2B, the upper vane ring (UVR) (26) and the lower vane ring (LVR) (28) are held together by studs or bolts (42), sometimes with nuts (44), which serve to apply a clamp load on the vane rings (26, 28), and on a plurality of spacers (46) placed between the vane rings (26, 28), such that the length of the spacer (46) determines the distance between the UVR (26) and the LVR (28), and thus the clearance between the cheeks (36) of the vanes (24) and the inner surfaces (38, 40) of the vane rings (26, 28). The bolts or studs (42) also serve to provide the angular orientation of the apertures in which the axles (30) of the vanes (24) are constrained. When studs are used, quite often the stud is screwed into the turbine housing (14), and the vane pack is assembled directly onto the turbine housing (14). However, studs are difficult to secure so that they do not unscrew with vibration, especially in situations where there are high temperatures (from 740° C. to 1050° C.). Similarly, in a situation where the temperature can range from below freezing to high combustion-like temperatures (from 740° C. to 1050° C.), it is difficult to maintain clamp load under a nut so that the nut does not come loose due to the differences in coefficients of thermal expansion between the materials of the components in the clamp load set.
Additionally, when cylindrical spacers (46) are used to determine and maintain the spacing between the UVR (26) and the LVR (26), the flow of gas around these typically cylindrical spacers (46) causes an aerodynamic phenomenon called vortex shedding, in which the flow periodically separates from the downstream side of the cylinder in a make and break cycle which can build to a resonance in the flow. Vortex shedding can cause a potentially damaging aerodynamically induced cyclic vibration in the thin blades of the turbine wheel (12).
During the assembly of the vane pack, much effort is expended ensuring that the correct components are used in the correct orientation and that the correct clamp loads are applied. Transport of the loose assembly is always difficult as removal of the unison ring can allow the individual sliding blocks to change their orientation relative to the slots in the unison ring making it quite difficult to re-assemble.
In the typical VTG vane pack, the upper end of the vane axle (30) is welded to the vane arm (34), a process which is costly in terms of equipment and time. Because the parts involved (vanes and vane arms) must endure high temperature and often corrosive by-products of engine combustion, they are typically fabricated from high nickel exotic materials which must be welded in an inert atmosphere. In mass production, this welding process requires substantial capital equipment investment. Because all air must be purged from close proximity of the parts being welded the process is often quite time consuming, adding at least 90 seconds to the manufacturing time.
Thus, there is a need for a vane pack configuration that allows efficient assembly, handling, transport and/or installation into a turbine housing.